Microbial infections are considered a major health problem with a growing concern toward those that do not respond to treatment due to antibiotic-resistant bacteria. According to the U.S. Centers for Disease Control and Prevention, approximately two million people are infected annually with bacteria resistant to antibiotics, of which ca. 23000 people die as a direct result of these infections. The prevention and treatment of these infections has drawn considerable attention and presents a critical challenge to develop drugs, antibiotics and/or antibacterial substances able to inhibit bacterial growth.
Infections due to Gram-negative Pseudonomas aeruginosa (P. Aeruginosa) and Gram-positive Staphylococcus aureus (S. aureus) have been documented in surgical sites, where they have been attributed to dermal injuries and burn wounds. Gram-negative bacteria are characterized by a lipid-rich outer membrane as well as a plasma membrane and a thin peptidoglycan layer, while Gram-positive bacteria are enshrouded in thicker, more resilient cell walls. This difference in cell wall is important for antibiotic development, since bacterial resistance might be due in part to cell wall composition.
Several conventional antibacterial agents, such as: tetracycline, streptomycin, and sulfonamides, have been developed to inhibit bacterial infections via the formation of biofilms. However, these antibiotics failed to inhibit all types of bacteria and multidrug-resistant strains have appeared due to the pathogen's evolution in counteracting the biocidal action of the agent molecules. Consequently, it is necessary to develop antibiotics that can overcome the limitations of the conventional antibacterial agents and preferably work against both types of bacteria. Interestingly, materials such as: silver nanoparticles, copper oxide, carbon nanomaterials, and metal oxide nanoparticles, have been reported as antimicrobial agents, and have been considered for use in wound infections, and in the clothing industry. In specific, silver is widely recognized for its capacity to kill bacteria. The mechanism of silver as an antibacterial agent is not totally clear, but it includes some possible mechanisms, such as: denaturation of the 30 s ribosomal subunits, inhibition of respiratory enzymes, binding and dimerization of RNA and DNA, and disruption of the outer membrane. Nevertheless, silver has shown high levels of toxicity at adequate concentrations for its antibacterial use.
The synthesis of silver composites might be a potential solution to overcome the negative side effects of silver, but optimizing the concentration of Ag in these composites remains a challenge. One approach to synthesize silver composites is to use carbon-based nanomaterials such as: graphene, carbon nanotubes, and graphene oxide with low concentrations of silver. The synthesis and use of silver-carbon nanotube complexes as antibacterial material has been previously reported. Another study reported that carbon nanotubes might be toxic due to their contamination with metallic catalysts used in their synthesis.
Graphene quantum dots (GQDs) are among the carbon nanostructures that may be good candidates for biomedical applications due to their solubility in aqueous solutions and high biocompatibility. GQDs are nanostructures of graphene in the size range of 2-20 nm with a set of excellent and unique chemical and physical properties. In general, GQDs have no apparent toxicity in vivo, and they have demonstrated high potential for utilization in cellular imaging, as antibacterial material, and drug delivery.